A conventional microminiature power converter, such as a DC-DC converter employed in a micro power supply, has a power supply IC chip mounted on an inductor by flip chip bonding or adhesion with an adhesive, connected by gold lines (bonding wires), and sealed with a mold resin such as an epoxy resin. Such an inductor is illustrated in FIGS. 30A, 30B, 30C, which show a structure of a conventional inductor 500. FIG. 30A is a plan view of an essential part of the inductor 500, FIG. 30B is a sectional view taken along the line 30B-30B of FIG. 30A, and FIG. 30C is a side view of the part A of FIG. 30A.
The inductor 500 is composed of a ferrite substrate 51, first and second coil conductors 54, 55, first connection conductors 56, first and second external electrodes 57, 58, and second connection conductors 59. A solenoid coil is formed in a central region of the ferrite substrate 51. A plurality of external electrodes are formed in the peripheral region of the ferrite substrate 51 surrounding the coil. The coil is composed of first coil conductors 54 on a front side (also referred to as a front surface side) of the ferrite substrate 51, second coil conductors 55 on a back side (also referred to as a back surface side) of the ferrite substrate 51, and first connection conductors 56 that are formed on a side wall of first through-holes 52 and connecting the coil conductors 54, 55. The external electrodes are arranged in the peripheral region of the ferrite substrate surrounding the coil and extending to the edge of the ferrite substrate 51. The external electrodes are composed of first external electrodes 57 formed on the front side of the ferrite substrate 51 and second external electrodes 58 formed on the back side of the ferrite substrate 51 at the places corresponding to the first external electrodes. The first and second external electrodes 57, 58 are connected by second connection conductors 59 formed on the side wall of the second through-holes 53. Each of the first connection conductor 56 and the second connection conductor 59 is surrounded by the ferrite substrate 51.
Japanese Unexamined Patent Application Publication No. 2004-274004, which corresponds to U.S. Pat. No. 6,930,584 B2, discloses a microminiature power converter having a power supply IC chip mounted on a coil substrate by flip chip bonding. This reference discloses that an inductance value can be increased by setting the length of the coil conductor constructing a planar type solenoid coil at a value larger than a predetermined value with respect to the width of the magnetic insulating substrate (a ferrite substrate). The front side of the ferrite substrate 51 is covered by an epoxy resin 60.
Each inductor 500, as shown in FIGS. 30A, 30B, 30C, is formed by cutting the first and second external electrodes 57, 58, the ferrite substrate 51, and the epoxy resin 60 along a scribe or cutting line. In that process, if the width W0 of the first and second external electrodes 57, 58 is wide, a flash 63 is generated, as shown in FIG. 31, at the second external electrode 58, which is not covered by epoxy resin, while such a flash is not created at the first external electrode 57, which is fixed with an epoxy resin 60. If the second external electrodes 58 with the flash 63 are soldered with a solder 64 to a packaging substrate 71, a solder bridge 65 is formed between the adjacent external electrodes 58 through the flash 63, short-circuiting the second external electrodes 58, as shown in FIG. 32. FIGS. 31 and 32 are side views of the part A of FIG. 30A. In FIG. 32, the dotted lines 66 show the configuration of the solder when no flash is generated, and the reference numeral 72 indicates a wiring on the packaging substrate.
In the device of the above-identified reference, external electrodes extend to the edge of the ferrite substrate like the structure shown in FIGS. 30A, 30B, 30C. But, the connection conductor connecting the external electrodes on the front and back surfaces is exposed to the edge of the ferrite substrate, which is different from the structure in FIGS. 30A, 30B, 30C. In that case too, the external electrodes causes generation of flashes in the cutting process along a scribe line like the structure of FIGS. 30A, 30B, 30C.
Accordingly, there remains a need to solve the above problem and provide an inductor having external electrodes that does not cause generation of flashes. The present invention address this need.